JP6384871B2 - Mode multiplexer / demultiplexer - Google Patents

Description

  The present invention relates to a mode multiplexer / demultiplexer necessary for mode multiplexing transmission, which is a multiplexing method in which each propagation mode is used as a carrier in an optical fiber cable having five or more propagation modes.

  Currently, traffic in optical fiber networks is increasing, and transmission capacity can be increased by using various methods such as increasing transmission speed, increasing the number of wavelength division multiplexing using wavelength division multiplexing (WDM) technology, and multi-level modulation. I have been trying to expand. However, it is expected that it will be difficult to expand the transmission capacity using existing transmission lines and conventional transmission methods in the future, so the expansion of the wavelength range, new transmission fibers, and new transmission methods have been studied. Yes.

  As for a new transmission fiber, a fiber structure has been proposed in which the effective area (Aeff) can be increased in order to suppress waveform distortion due to fiber nonlinearity. Fiber nonlinear suppression leads to an increase in input power that can be input to the fiber. If the input power can be increased, advantages such as higher transmission speed and further multi-value can be obtained. However, as shown in Non-Patent Document 1, since the expansion of Aeff is based on a single mode operation, there is a trade-off relationship between bending loss and single mode operation, and it is difficult to significantly increase Aeff. There is.

  Regarding a new transmission method, a multiplexing method using a propagation mode of an optical fiber has been proposed in Patent Document 1, but a method for exciting a desired higher-order mode has not been proposed, and it is used at a single wavelength. Therefore, there is a problem that it is difficult to realize a large capacity. As a mode demultiplexer for using the propagation mode of an optical fiber, a method of demultiplexing a mode by changing a light receiving position as shown in Non-Patent Document 2 has been proposed. In such a situation, crosstalk between modes becomes large, and the design of the optical receiver becomes complicated. Therefore, it is not preferable for use in mode multiplex transmission using a propagation mode as a carrier.

JP-A-8-288911

Matsui et al., “Single-mode photonic fiber with low bending loss and Aeff of> 200 μm2 for ultra high-speed WDM transmission”, OFC2010, PD. C. P. Tsekrekos et al., “Mode-selective spatial filtering for in- cluded robustness in a mode group diversity multiplexing”, OPTIC LETTERS, Vol. 32, no. 9, 2007. R. Ryf et al., “Optical Coupling Components for Spatial Multiplexing in Multi-Mode Fibers”, ECOC2011, Th. 12 B. 1. N. Hanzawa et al., “Four-mode plc-based mode multi / demultiplexer with LP11 mode rotator on one chip for MDM transmission”, ECOC2014, We. 1.1.1 N. Hanzawa et al. , “Asymmetric parallel waveguide with mode conversion for mode and wavelength division multiplexing transmission,” OFC2012, paper Otu1l. 4 (2012).

  In mode multiplex transmission using an optical fiber with multiple propagation modes and each propagation mode of the optical fiber as a carrier, the existing device is assumed to operate in the basic mode. It is difficult to realize mode multiplexing transmission by using. In addition, the mode multiplexer / splitters proposed so far have poor demultiplexing efficiency, making long-distance transmission and high-speed signal transmission difficult, and using a spatial system makes the configuration complicated. There are challenges. In the method using the spatial system, it is possible to achieve high demultiplexing efficiency for each mode as described in Non-Patent Document 3, but the insertion loss is as large as 8 dB or more, and it is difficult to reduce the size of the device. There are also other issues.

  In recent years, as shown in Non-Patent Document 4, a four-mode multiplexer / demultiplexer using an asymmetric parallel waveguide has been proposed. By using an asymmetric parallel waveguide, signals can be handled only at the fundamental mode at the transmitting and receiving ends. It is possible to use an existing device as it is. However, in order to realize further large-capacity transmission, it is desirable to increase the number of mode multiplexing. However, the multiplexer / demultiplexer described in Non-Patent Document 4 is limited to four modes, and has five or more modes. No multiplexing / demultiplexing method for realizing multiplexing has been proposed.

  Therefore, an object of the present invention is to provide a mode multiplexer / demultiplexer that can use an existing device and can perform mode multiplex transmission of five or more modes using an optical fiber.

  The mode multiplexer / demultiplexer according to the present invention is a mode of the fifth and sixth modes by adding a taper portion whose waveguide width continuously changes in the longitudinal direction to the multiplexer / demultiplexer using the parallel waveguide. An LP21b mode and an LP02 mode are generated.

Specifically, one mode multiplexer / demultiplexer according to the present invention is:
a main waveguide having a tapered portion capable of propagating a mode of n (n is an integer of 5 or more) or more and having a waveguide width continuously changing in the longitudinal direction;
A sub-waveguide having a coupling portion that is non-contact with the main waveguide, generates mode conversion with the main waveguide, and transfers optical power;
A mode multiplexer / demultiplexer comprising:
The sub-waveguide has an effective refractive index of a propagating mode for the desired wavelength range of light, and has three peaks in one of the horizontal direction and the vertical direction with respect to the substrate, which propagates through the main waveguide. , Consistent with the effective refractive index of the intermediate mode of the field distribution with one peak on the other,
The tapered portion is characterized by adiabatic conversion between a field distribution mode corresponding to the LP21b mode or LP02 mode, which is a propagation mode in an optical fiber, and the intermediate mode.

  This mode multiplexer / demultiplexer shifts the fundamental mode light propagating in the sub-waveguide from the coupling portion to the main waveguide in the multiplexing direction to generate an intermediate mode of the E31 mode or E13 mode in the main waveguide, and the tapered portion. Thus, the intermediate mode is converted to the LP21b mode or LP02 mode. In the multiplexer / demultiplexer using the parallel waveguide, the demultiplexing operation is the reverse of the multiplexing, so this mode multiplexer / demultiplexer transmits the LP21b mode or LP02 mode light propagating through the main waveguide to the sub-waveguide. Transition to basic mode is possible. The conversion between the intermediate mode and the LP21b mode or LP02 mode can be realized by adjusting the structure of the tapered portion.

  Therefore, the present invention can provide a mode multiplexer / demultiplexer that can use an existing device and enables five-mode mode multiplexing transmission using an optical fiber.

Further, another mode multiplexer / demultiplexer according to the present invention is
a main waveguide having a first tapered portion capable of propagating a mode of n (n is an integer of 6 or more) or more and having a waveguide width continuously changing in the longitudinal direction;
A sub-waveguide having a coupling portion that is non-contact with the main waveguide, generates mode conversion with the main waveguide, and transfers optical power;
A mode multiplexer / demultiplexer comprising:
The sub-waveguide has an effective refractive index of a propagating mode for the desired wavelength range of light, and three peaks in the horizontal direction and one peak in the vertical direction propagating through the main waveguide. Or the effective refractive index of the second intermediate mode of the field distribution having three peaks in the vertical direction and one peak in the horizontal direction with respect to the substrate. And
The first taper section performs adiabatic conversion between a field distribution mode corresponding to one of LP21b mode and LP02 mode, which is a propagation mode in an optical fiber, and the first intermediate mode, and a field distribution mode corresponding to the other mode. The second intermediate mode is subjected to adiabatic conversion.

  Although this mode multiplexer / demultiplexer is the same as the first mode multiplexer / demultiplexer, this mode multiplexer / demultiplexer generates two kinds of intermediate modes (for example, E31 mode and E13 mode) in the main waveguide. Then, these intermediate modes are converted into the LP21b mode and the LP02 mode, respectively, at the first taper portion. Therefore, the present invention can provide a mode multiplexer / demultiplexer that can use an existing device and enables 6-mode mode multiplexing transmission using an optical fiber.

  In the main waveguide of another mode multiplexer / demultiplexer according to the present invention, a waveguide width continuously changes in a longitudinal direction between the coupling portions of two sub waveguides, and the first intermediate mode and It further has a 2nd taper part which carries out adiabatic conversion mutually with the 2nd middle mode.

  Mode conversion between intermediate modes is possible by adjusting the structure of the tapered portion of the waveguide. For this reason, the E31 mode is generated at the coupling portion at the front stage, converted to the E13 mode at the second taper portion, the E31 mode is further generated at the coupling portion at the rear stage, and finally the E13 mode is changed to the LP21b mode at the first taper portion. If the E31 mode is converted to the LP02 mode, 6-mode mode multiplex transmission is possible.

The mode multiplexer / demultiplexer according to the present invention includes m coupling units that are non-contact with the main waveguide and the sub-waveguide, generate mode conversion with the main waveguide, and transfer optical power. m is an integer of 3 or more) side waveguides,
In the side waveguide, the effective refractive index of the propagating mode exclusively coincides with the effective refractive indexes of the three higher-order modes on the lower-order side of the main waveguide with respect to a desired light wavelength range. It is characterized by.

  This mode multiplexer / demultiplexer has three higher-order modes (LP11a, LP11b, LP21a) on the low-order side in the required number of side waveguides before the intermediate mode is generated in the main waveguide at the coupling portion of the sub-waveguide. Can be generated.

  The at least one coupling portion of the mode multiplexer / demultiplexer according to the present invention may be at least one intermediate waveguide that is not in contact with the main waveguide, the sub-waveguide, and the side waveguide.

  The main waveguide, the sub-waveguide, and the side waveguide of the mode multiplexer / demultiplexer according to the present invention have the same waveguide height, and at least the main waveguide, the sub-waveguide, and the side waveguide One can include a mode rotator that converts the LP11a mode to the LP11b mode.

In the mode multiplexer / demultiplexer according to the present invention, the intermediate mode is an E31 mode,
In the direction of multiplexing,
LP11a mode, LP11b mode, and LP21a mode are combined from the coupling part of the side waveguide to the main waveguide through which the LP01 mode propagates, and further, E31 mode is multiplexed from the coupling part of the sub-waveguide, The taper portion can convert the E31 mode to the LP02 mode.

In the mode multiplexer / demultiplexer according to the present invention, the first intermediate mode is an E31 mode, and the second intermediate mode is an E13 mode,
In the direction of multiplexing,
The LP11a mode, the LP11b mode, and the LP21a mode are combined from the coupling portion of the side waveguide to the main waveguide through which the LP01 mode propagates, and the E31 mode is multiplexed from the coupling portion of one of the sub waveguides. The E31 mode is converted to the E13 mode by the second taper part, and the E31 mode is multiplexed from the coupling part of the other sub-waveguide, and the E31 mode is changed to the LP02 mode by the first taper part. Can be converted to LP21b mode.

  INDUSTRIAL APPLICABILITY The present invention can provide a mode multiplexer / demultiplexer that can use an existing device and enables mode multiplexing transmission of five or more modes using optical fibers.

It is a figure explaining the mode multiplexer / demultiplexer which multiplexes / demultiplexes LP01 mode and LP11 mode. It is a figure explaining the electric field distribution in the waveguide of each mode. It is a figure explaining the effective refractive index and electric field distribution of the 5th mode and 6th mode in a waveguide when changing the width | variety of a waveguide. It is a figure explaining the structure of a taper part. It is a figure explaining the structure of the taper part for obtaining LP21b and LP02 mode simultaneously. It is a figure explaining the structure of the mode multiplexer / demultiplexer which concerns on this invention. It is a figure explaining the structure of the mode multiplexer / demultiplexer which concerns on this invention. It is a figure explaining the structure of the mode multiplexer / demultiplexer which concerns on this invention. It is a figure explaining the structure of the mode multiplexer / demultiplexer which concerns on this invention. It is a figure explaining the structure of the mode multiplexer / demultiplexer which concerns on this invention. It is a figure explaining the design of a coupling part. It is a figure explaining the structure of the mode multiplexer / demultiplexer which concerns on this invention. It is a figure explaining the design of a coupling part.

  Embodiments of the present invention will be described with reference to the accompanying drawings. The embodiments described below are examples of the present invention, and the present invention is not limited to the following embodiments. In the present specification and drawings, the same reference numerals denote the same components.

  In this embodiment, a fundamental mode (LP01), a first higher-order mode (LP11a, LP11b), a second higher-order mode (LP21a, LP21b), and a third higher-order mode (LP02), which are propagation modes in an optical fiber, are used. The 6-mode multiplex transmission used is assumed.

  FIG. 1 is a diagram illustrating a mode multiplexer / demultiplexer that multiplexes / demultiplexes the LP01 mode and the LP11 mode. In this example, the relative refractive index difference Δ is equal and the height of the waveguide is a plurality of waveguides, and the parallel waveguide propagates in one of the waveguides by the adjacent coupling portion. Designed to convert a particular mode into a particular mode propagating in the other waveguide.

  According to Non-Patent Document 5, mode coupling can be generated by appropriately designing the height and width of two waveguides and matching the effective refractive indexes of modes propagating through the two waveguides. In addition, by appropriately designing the interaction length, light propagating in one waveguide can be transferred to the other waveguide.

  Further, when the two waveguides have the same height, it is stated that the mode that can be converted from the LP01 mode is the LP11a mode, and by using the LP11 mode rotator described in Non-Patent Document 4, 4-mode (LP01, LP11a, LP11b, LP21a) multiplexing / demultiplexing can be realized.

  In order to multiplex / demultiplex five or more modes, it is necessary to convert the LP01 mode to the LP21b which is the fifth mode or the LP02 mode which is the sixth mode. FIG. 2 shows the electric field distribution in the waveguide of each mode. Here, the width and height of the waveguide are the same.

  When the width and height of the waveguide are the same, it can be seen that the electric field distribution in the sixth mode is the same as that in the LP02 mode in the optical fiber. On the other hand, as shown in the figure, it can be seen that when the width and height are different, the electric field distribution is different from the LP02 mode of the optical fiber. That is, in order to multiplex the sixth mode and propagate it as the LP02 mode of the optical fiber, the height and width of the waveguide must be matched.

  However, under the conditions where the height and width of the waveguide are the same, the effective refractive indexes of the LP21b mode and the LP02 mode are almost the same, which cannot be dealt with by the methods studied so far. Therefore, a 6-mode multiplexer / demultiplexer is realized using a taper structure.

(Embodiment 1)
FIG. 3 is a diagram for explaining the effective refractive index and electric field distribution of the fifth mode and the sixth mode in the waveguide (that is, the tapered portion) when the width of the waveguide is changed. The height of the waveguide is 12 μm and the relative refractive index difference is 0.7%. When the waveguide is not square, the effective refractive index of the two modes is different, but the electric field distribution is greatly different from that of the LP02 mode guiding the optical fiber, which is not preferable. On the other hand, in the case of a square, an electric field distribution similar to that of the LP02 mode is obtained, but the effective refractive indexes of the two modes are close to each other, and it is difficult to couple each mode from another waveguide. I understand.

  FIG. 4 is a diagram illustrating the structure of the tapered portion. For example, when the waveguide width is W and the waveguide height is H, the waveguide structure on the incident side is W <H and the waveguide structure on the output side is continuous in the longitudinal direction so that W> H. Assume a tapered structure in which W changes. The mode of light propagating through such a tapered structure changes along the effective refractive index curve in FIG. 3 based on the principle of adiabatic conversion by the tapered structure. That is, when the mode (E31) having the electric field distribution shown in the figure is excited, the LP21b mode can be obtained in the structure of W = H, and when the structure is changed to the structure of W> H, it is converted into the E13 mode in the waveguide. The

  In the tapered portion, it is necessary to make the taper length sufficiently long in order to perform adiabatic conversion, but as shown in FIG. 4, the taper length can be shortened by combining a plurality of taper structures.

  FIG. 5 is a diagram for explaining the structure of the tapered portion for simultaneously obtaining the LP21b and LP02 modes. As shown in FIG. 3, LP21b and LP02 modes can be obtained by exciting two modes with a structure of W ≠ H and setting W = H with a tapered structure.

  Both the up-taper structure that increases W from the structure of W <H as shown in FIG. 4 and the down-taper structure that decreases W from the structure of W> H shown in FIG. Can be used.

  A specific configuration of the mode multiplexer / demultiplexer will be described below. In the multiplexer / demultiplexer using the parallel waveguide, the demultiplexing operation is the reverse of the multiplexing operation.

(Embodiment 2)
6 and 7 are diagrams illustrating the mode multiplexer / demultiplexer (301, 302) including the tapered portion described in the first embodiment. The mode multiplexer / demultiplexer (301, 302)
a main waveguide 10 capable of propagating a mode of n (n is an integer of 6 or more) and having a first tapered portion 41 in which the waveguide width continuously changes in the longitudinal direction;
A sub-waveguide (21, 22) having a coupling portion (31, 32) that is non-contact with the main waveguide 10, generates mode conversion with the main waveguide, and shifts optical power;
A mode multiplexer / demultiplexer provided on the substrate 1,
The sub-waveguides (21, 22) have an effective refractive index of a propagating mode with respect to a desired wavelength range of light. The effective refractive index of the first intermediate mode of the field distribution having one peak in the field distribution, or the effective refractive index of the second intermediate mode of the field distribution having three peaks in the vertical direction and one peak in the horizontal direction with respect to the substrate 1 And
The first taper portion 41 adiabatically converts a field distribution mode corresponding to one of the LP21b mode and LP02 mode, which is a propagation mode in an optical fiber, and the first intermediate mode, and a field distribution mode corresponding to the other mode. The second intermediate mode is subjected to adiabatic conversion.
6 and 7, the main waveguide 10 is described as “first waveguide”, the sub-waveguide 21 is described as “second waveguide”, and the sub-waveguide 22 is described as “third waveguide”. Further, the first taper portion 41 in FIG. 6 is described as an “up taper portion” in which the waveguide width is widened in the light combining direction, and the first taper portion 42 in FIG. 7 is in the light combining direction. The waveguide width is narrowed and is described as “down taper portion”.

  For example, as shown in FIG. 6, in the coupling portion 31 of the first waveguide (main waveguide 10) and the second waveguide (sub waveguide 21), the fundamental mode propagating through the second waveguide is peaked in the vertical direction. Has been propagated through the third waveguide at the coupling portion 32 between the first waveguide (main waveguide 10) and the third waveguide (sub-waveguide 22). The fundamental mode is converted into a first intermediate mode (E31 mode) having three peaks in the lateral direction, and finally, the LP21b mode and the LP02 mode can be obtained by the tapered portion 41 of the main waveguide 10.

  In addition, as shown in FIG. 7, in the coupling portion 31 of the first waveguide (main waveguide 10) and the second waveguide (sub-waveguide 21), the fundamental mode propagating through the second waveguide is peaked in the lateral direction. Has been propagated through the third waveguide at the coupling portion 32 of the first waveguide (main waveguide 10) and the third waveguide (sub-waveguide 22). The fundamental mode is converted into a second intermediate mode (E13 mode) having three peaks in the vertical direction, and finally, the LP21b mode and the LP02 mode can be obtained by the tapered portion 42 of the main waveguide 10.

In FIGS. 6 and 7, the LP21b mode and the LP02 mode are obtained with two sub-waveguides. However, in order to obtain the LP21b mode or the LP02 mode, light may be multiplexed with one sub-waveguide. That is, the mode multiplexer / demultiplexer
a main waveguide having a tapered portion capable of propagating a mode of n (n is an integer of 5 or more) or more and having a waveguide width continuously changing in the longitudinal direction;
A sub-waveguide having a coupling portion that is non-contact with the main waveguide, generates mode conversion with the main waveguide, and transfers optical power;
A mode multiplexer / demultiplexer comprising:
The sub-waveguide has an effective refractive index of a propagating mode for the desired wavelength range of light, and has three peaks in one of the horizontal direction and the vertical direction with respect to the substrate, which propagates through the main waveguide. , Consistent with the effective refractive index of the intermediate mode of the field distribution with one peak on the other,
The tapered portion is characterized by adiabatic conversion between a field distribution mode corresponding to the LP21b mode or LP02 mode, which is a propagation mode in an optical fiber, and the intermediate mode.

(Embodiment 2)
However, as described in Non-Patent Document 4, even if the effective refraction is matched between the two modes, they are not coupled if the overlap integral of the electric field distribution is zero. That is, if the waveguide height is constant, the LP01 mode and the LP11a mode can be coupled to the E31 mode, but cannot be coupled to the E13 mode because the overlap integral is zero.

Therefore, the present embodiment is configured as shown in FIGS. That is, the mode multiplexer / demultiplexer 303 in FIG. 8 includes the main waveguide 10 of the mode multiplexer / demultiplexer 302 in FIG. 7 between the coupling portions (31, 32) of the two sub waveguides (21, 22). The waveguide width is continuously changed in the longitudinal direction, and further includes a second taper portion 43 that performs adiabatic conversion between the first intermediate mode and the second intermediate mode. Further, the mode multiplexer / demultiplexer 304 in FIG. 9 includes the main waveguide 10 of the mode multiplexer / demultiplexer 301 in FIG. 6 between the coupling portions (31, 32) of the two sub waveguides (21, 22). The waveguide width is continuously changed in the longitudinal direction, and further includes a second taper portion 44 that performs adiabatic conversion between the first intermediate mode and the second intermediate mode.
The second taper portion 43 in FIG. 8 is described as an “up taper portion” in which the waveguide width is widened in the light combining direction, and the second taper portion 44 in FIG. 9 is a waveguide in the light combining direction. The width is narrowed and is described as “down taper part”.

  FIG. 8 shows an example of combining the LP21b and LP02 modes when the waveguide height is constant. After converting from the fundamental mode of the second waveguide to the first intermediate mode (E31 mode) at the coupling portion 31 of the first waveguide (main waveguide 10) and the second waveguide (sub-waveguide 21), the up taper portion 43 converts the E31 mode into the second intermediate mode (E13 mode).

  Also in the coupling portion 32 between the first waveguide (main waveguide 10) and the third waveguide (sub-waveguide 22), the fundamental mode of the third waveguide is converted to the first intermediate mode (E31 mode), and finally In the down taper portion 42, the E13 mode and the E31 mode are converted into the LP21b mode and the LP02 mode, respectively.

The mode multiplexer / demultiplexer 304 in FIG. 9 is obtained by reversing the taper structure of the mode multiplexer / demultiplexer 303 in FIG. That is, the first taper portion 42 having the down taper structure is the first taper portion 41 having the up taper structure, and the second taper portion 43 having the up taper structure is the second taper portion 44 having the down taper structure.
Similarly to the description of the mode multiplexer / demultiplexer 303 in FIG. 8, the mode multiplexer / demultiplexer 304 in FIG. 9 converts the E31 mode into the E13 mode by the second taper portion 44, and then the first taper together with the combined E31 mode. The unit 41 converts each into LP21b mode and LP02 mode.

(Embodiment 3)
FIG. 10 is a diagram for explaining the mode multiplexer / demultiplexer 305 of the present embodiment. The mode multiplexer / demultiplexer 305 is a 6-mode multiplexer / demultiplexer when the heights of all the waveguides are constant. The mode multiplexer / demultiplexer 305 is obtained by adding a side waveguide that generates a lower-order higher-order mode (LP11a, LP11b, LP21a) to the mode multiplexer / demultiplexer 303 in FIG. That is, the mode multiplexer / demultiplexer 305 is non-contact with the main waveguide 10 and the sub-waveguides (21, 22), and has a coupling unit that generates mode conversion with the main waveguide 10 and transfers optical power. m side waveguides (51, 52, 53) are further provided (m is an integer of 3 or more),
In the side waveguides (51, 52, 53), the effective refractive index of the propagating mode is exclusive of the effective refractive indexes of the three higher-order modes on the lower-order side of the main waveguide 10 with respect to the desired wavelength range of light. It is characterized by matching.

  The mode multiplexer / demultiplexer 305 includes at least one intermediate conductor in which at least one of the coupling portions is not in contact with the main waveguide 10, the sub waveguide (21, 22), and the side waveguide (51, 52, 53). Waveguide 61.

  The main waveguide 10, the sub waveguides (21, 22), and the side waveguides (51, 52, 53) have the same waveguide height, and the main waveguide 10, the sub waveguides (21, 22), and the side waveguides. At least one of the waveguides (51, 52, 53) includes a mode rotator that converts the LP11a mode to the LP11b mode.

  As described in Non-Patent Document 4, the multiplexing / demultiplexing of the LP01, LP11a, LP11b, and LP21a modes under the condition that the waveguide height is constant is realized by using a mode rotator capable of converting from the LP11a mode to the LP11b mode. it can. The mode multiplexer / demultiplexer 305 has a mode rotator 72 and a mode rotator 71 in the main waveguide 10 and the intermediate waveguide 61, respectively, and LP11a that is mode-converted and coupled from the side waveguide 51 to the intermediate waveguide 61. The mode is converted to the LP11b mode by the mode rotator 71, and the LP11a mode that is mode-converted from the side waveguide 52 to the main waveguide 10 and converted is converted to the LP11b mode by the mode rotator 72. At this time, the LP01 mode and LP11b are propagated through the main waveguide 10, and the LP11a mode converted from the side waveguide 53 and the LP21a mode converted from the intermediate waveguide 61 are coupled. That is, the mode multiplexer / demultiplexer 305 can multiplex / demultiplex the modes of LP01, LP11a, LP11b, and LP21a at the coupling units (33, 34, 35).

  As described with reference to the mode multiplexer / demultiplexer 303 in FIG. 8, the mode multiplexer / demultiplexer 305 converts the fundamental mode of the sub-waveguide 21 into the E31 mode in the coupling unit 31, and the tapered unit 43 of the up taper structure. This is converted to the E13 mode, and further converted by the tapered portion 42 of the down taper structure to obtain the LP21b mode. The mode multiplexer / demultiplexer 305 converts the fundamental mode of the sub-waveguide 22 into the E31 mode in the coupling unit 32, and further converts this in the tapered unit 42 having the down-taper structure to obtain the LP02 mode.

  As described above, the mode multiplexer / demultiplexer 305 is capable of multiplexing / demultiplexing six modes of LP01, LP11a, LP11b, LP21a, LP21b, and LP02.

  In order to realize a five-mode multiplexer / demultiplexer, the fifth mode may be the LP21b or LP02 mode, and it is easy to remove one of the sub-waveguides (21 or 22) from the structure of FIG. Can be realized. Note that the mode rotors (71, 72) are not essential if the waveguides are manufactured to have different heights.

  FIG. 11 is a design example of each coupler (coupler 1, coupler 2, coupler 3, and coupler 4) of the mode multiplexer / demultiplexer 305 in FIG. 11A illustrates coupler 1, FIG. 11B illustrates coupler 2, FIG. 11C illustrates coupler 3, and FIG. 11D illustrates coupler 4. By appropriately designing the width and height of the waveguide, the waveguide interval g, and the coupling length L, a coupling efficiency of 98% or more can be realized.

  In the configuration of FIG. 10, the width w1 of the sub waveguide in the coupler 3 (coupling portion 31) is 1.19 μm, which is small with respect to the height of the waveguide, which may be difficult to realize in manufacturing. In such a case, as shown in the mode multiplexer / demultiplexer 306 in FIG. 12, the basic mode of the sub-waveguide 21 may be temporarily converted to the LP11a mode using the coupling unit 36 (coupler 1). FIG. 13 is a design example of the coupling unit 31 (coupler 3) of the mode multiplexer / demultiplexer 306. By once converting to LP11a using the coupling portion 36, the width w1 of the sub-waveguide can be increased to 6.2 μm.

[Appendix]
The following describes the mode multiplexer / demultiplexer of this embodiment.

(1):
a main waveguide capable of propagating n or more modes (n is an integer of 5 or more);
M sub-waveguides (m is an integer of 1 or more) having a coupling portion that is non-contact with the main waveguide, generates mode conversion with the main waveguide, and shifts optical power;
A mode multiplexer / demultiplexer comprising:
In the sub-waveguide, the effective refractive index of the propagating mode exclusively matches the effective refractive index of the n−1 modes of the main waveguide with respect to a desired wavelength range of light,
Coupled to at least one mode A having a field distribution with three peaks in the horizontal or vertical direction of the waveguide including the waveguide center and one peak in the vertical or horizontal direction;
A mode multiplexer / demultiplexer characterized in that mode A is converted to LP21b or LP02 in a tapered portion where the waveguide width continuously changes.

(2):
In the mode multiplexer / demultiplexer, the mode A coupled from the sub waveguide is
E31 mode having a field distribution with three peaks in the horizontal direction and one peak in the vertical direction,
A first taper portion for converting the E31 mode into an E13 mode having a field distribution with three peaks in the vertical direction of the waveguide and one peak in the horizontal direction;
The mode multiplexer / demultiplexer according to (1), wherein in the second taper portion, the propagating E13 and E31 modes are converted into LP21b and LP02 modes.

(3):
The mode multiplexer / demultiplexer according to (1) or (2) above, wherein the coupling portion is at least one intermediate waveguide that is not in contact with the main waveguide and the sub-waveguide.

(4):
The mode multiplexer / demultiplexer according to any one of (1) to (3) above, wherein the mode multiplexer / demultiplexer includes a mode rotator for converting the LP11a mode to the LP11b mode with a constant waveguide height. vessel.

(5):
In the mode multiplexer / demultiplexer,
A coupling unit coupling the LP01 mode and the LP11a mode;
A coupling unit coupling the LP11b mode and the LP21a mode;
A coupling unit coupling the LP01 or LP11a mode and the E31 mode;
With
In the main waveguide of the mode multiplexer,
The 5- or 6-mode multiplexer / demultiplexer according to (1) to (4) above, which multiplexes / demultiplexes a signal in which the LP21b or LP02 mode or a mode including both modes is multiplexed.

  The present invention can be applied to the information communication industry.

1: Substrate 10: Main waveguide (first waveguide)
21: Sub waveguide (second waveguide)
22: Sub-waveguide (third waveguide)
31 to 36: coupling portions 41 to 44: taper portions 51 to 53: side waveguide 61: intermediate waveguides 301 to 306: mode multiplexer / demultiplexer

Claims (8)

a main waveguide having a tapered portion capable of propagating a mode of n (n is an integer of 5 or more) or more and having a waveguide width continuously changing in the longitudinal direction;
A sub-waveguide having a coupling portion that is non-contact with the main waveguide, generates mode conversion with the main waveguide, and transfers optical power;
A mode multiplexer / demultiplexer comprising:
The sub-waveguide has an effective refractive index of a propagating mode for the desired wavelength range of light, and has three peaks in one of the horizontal direction and the vertical direction with respect to the substrate, which propagates through the main waveguide. , Consistent with the effective refractive index of the intermediate mode of the field distribution with one peak on the other,
A mode multiplexer / demultiplexer characterized in that the tapered portion adiabatically converts a field distribution mode corresponding to the LP21b mode or LP02 mode, which is a propagation mode in an optical fiber, and the intermediate mode. a main waveguide having a first tapered portion capable of propagating a mode of n (n is an integer of 6 or more) or more and having a waveguide width continuously changing in the longitudinal direction;
A sub-waveguide having a coupling portion that is non-contact with the main waveguide, generates mode conversion with the main waveguide, and transfers optical power;
A mode multiplexer / demultiplexer comprising:
The sub-waveguide has an effective refractive index of a propagating mode for the desired wavelength range of light, and three peaks in the horizontal direction and one peak in the vertical direction propagating through the main waveguide. Or the effective refractive index of the second intermediate mode of the field distribution having three peaks in the vertical direction and one peak in the horizontal direction with respect to the substrate. And
The first taper section performs adiabatic conversion between a field distribution mode corresponding to one of LP21b mode and LP02 mode, which is a propagation mode in an optical fiber, and the first intermediate mode, and a field distribution mode corresponding to the other mode. A mode multiplexer / demultiplexer, wherein the second intermediate mode is subjected to adiabatic conversion. The main waveguide is
A second taper portion between the coupling portions of the two sub-waveguides, wherein the waveguide width continuously changes in the longitudinal direction, and the first intermediate mode and the second intermediate mode are adiabatically converted from each other; The mode multiplexer / demultiplexer according to claim 2, further comprising: M side waveguides (m is an integer of 3 or more) having a coupling portion that is non-contact with the main waveguide and the sub-waveguide, generates mode conversion with the main waveguide, and shifts optical power. Further comprising
In the side waveguide, the effective refractive index of the propagating mode exclusively coincides with the effective refractive indexes of the three higher-order modes on the lower-order side of the main waveguide with respect to a desired light wavelength range. The mode multiplexer / demultiplexer according to any one of claims 1 to 3. 5. The mode multiplexing / demultiplexing according to claim 4 , wherein the at least one coupling portion is at least one intermediate waveguide that is not in contact with the main waveguide, the sub-waveguide, and the side waveguide. 6. vessel. The main waveguide, the sub-waveguide, and the side waveguide have the same waveguide height,
6. The mode multiplexer / demultiplexer according to claim 4, wherein at least one of the main waveguide, the sub-waveguide, and the side waveguide includes a mode rotator that converts an LP11a mode into an LP11b mode. The intermediate mode is E31 mode,
In the direction of multiplexing,
LP11a mode, LP11b mode, and LP21a mode are combined from the coupling part of the side waveguide to the main waveguide through which the LP01 mode propagates, and further, E31 mode is multiplexed from the coupling part of the sub-waveguide, The mode multiplexer / demultiplexer according to any one of claims 4 to 6, wherein the E31 mode is converted to the LP02 mode at the taper portion. The first intermediate mode is an E31 mode, and the second intermediate mode is an E13 mode;
In the direction of multiplexing,
The LP11a mode, the LP11b mode, and the LP21a mode are combined from the coupling portion of the side waveguide to the main waveguide through which the LP01 mode propagates, and the E31 mode is multiplexed from the coupling portion of one of the sub waveguides. The E31 mode is converted to the E13 mode by the second taper part, and the E31 mode is multiplexed from the coupling part of the other sub-waveguide, and the E31 mode is changed to the LP02 mode by the first taper part. 7. The mode multiplexer / demultiplexer according to claim 4, wherein the mode multiplexer / demultiplexer is converted to the LP21b mode.